Information concerning the condition of a borehole is important for the success of the drilling process from both a quality control and planning viewpoint. The information, which comprises many parameters, may be used to warn the engineers of changes in well profile and the stability of the operation. For example, borehole diameters must be carefully controlled during the drilling as they can affect the performance of the downhole assemblies used in directional drilling, restrict the ability of the drilling fluid to remove cuttings from the well and may limit the success of cementing the production casings in place prior to commercial operation of the well. Further, borehole information is used to determine the formation types (lithology) encountered as an indication of the well's potential to produce hydrocarbons. There are many other applications in practice which can use timely wellhole information.
In order to obtain information about the conditions downhole, it is frequently necessary to suspend the drilling process at some specified depths, remove (extract) the drillstring from the wellhole and lower a sensing tool with a collection of sensors at the end of a cable (a wireline telemetering system) into the well. The sensor tool is then slowly withdrawn and the data from the tool is transmitted to the surface up the connecting cable. The information about the well condition is recorded (logged) and subsequently analyzed. This process is known as wireline logging and is capable of producing a tremendous amount of information which the engineers can the use to construct a physical representation of the condition of the well over its entire length.
This type of monitoring has two inherent problems: (1) it relies on gravity for the instrument to descend, and, therefore, if the hole is inclined or has shelf-like steps on the outer surface of the borehole, the instrument may get hung up, and; (2) it does not occur during normal drilling or tripping operations and does not, therefore, provide the driller with real-time or current information on the state of the drilling. Finally, in that drilling operations must be suspended, this method is time-consuming to the well drilling operations and is therefore expensive to undertake.
A second technique of logging while drilling (LWD) involves the positioning a specialized drill collar containing sensing devices near the drill bit. As it is located in the drillstring, it is able access horizontal sections of the wellbore and is not susceptible to hanging up. This technique telemeters information to the surface by acoustical pulses transmitted through the drilling fluid. This technique has been limited in a number of ways: Firstly, it has been limited by the types of drilling fluids that can provide effective acoustical coupling, often limited to drilling fluids such as water, oil or emulsions. Furthermore, as this technique obtains data while the drill bit is rotating (that is, a noisy and vibrating environment), it, typically, has a very slow data transmission rate (1 bit per second) that requires substantial computer processing to compensate for the rotation of the drill bit and artifactual errors.
Furthermore, LWD only collects data immediately behind the drilling bit and does not obtain data from other regions of the borehole. Therefore, if a washout occurs uphole, this technique will not detect it. It therefore becomes necessary to back-up LWD data with wireline logging data. Accordingly, this technique, in addition to requiring expensive LWD equipment further requires the time-consuming technique of wireline logging with additional wireline logging equipment.
A variety of techniques and methods have been used to transfer accumulated data from the sensor tools at the well bottom in the LWD application. One wireless technique transmits information to the surface using acoustic signalling through the drilling fluid (mud) as is called mud pulsing. This kind of telemetry, discussed in Canadian patent 1,098,202, is restricted to certain kinds of drilling fluid which exhibit reasonably low loss transmission. Nevertheless, transmission speeds are low (in the order of one bit per second) due to restricted bandwidth at the sensors and the attenuation constants of the medium. Data compression is used to reduce the number of transmitted bits in an effort to improve the system's performance but this is still fundamentally limited.
Efforts to improve upon the telemetry path by using the drillstring as the medium for acoustic signalling have proven to be only marginally successful. Canadian patent 1,098,202 and U.S. Pat. Nos. 4,139,836 and 4,320,473 have discussed this issue in depth but the technique has failed to gain support in the drilling industry.